The purpose of the Seek, Test, Treat, and Retain (STTR) Data Harmonization Coordinating Center (DCC) is to coordinate and support the use of an integrated harmonized data set to test the effectiveness of the STTR paradigm and to improve understanding of the intersection of HIV infection, drug abuse, and the HIV treatment cascade. This will require a DCC experienced in data harmonization, multi-site coordination, and research on drug abuse interventions and impact among persons living with HIV (PLWH) to comprehensively integrate clinical research data from multiple studies. Therefore, we propose the following aims: 1) Provide operational, administrative, and logistical support for the STTR data harmonization initiative 2) Identify and obtain access to STTR data from existing HIV sources. Inclusion of CNICS data has already been approved by the CNICS Executive Committee. 3) Work with a multi-disciplinary team of drug abuse and HIV experts, biostatisticians, epidemiologists, experts from NIDA, and other interested parties to develop and implement a state-of-the-art scientific agenda, including a dissemination and publication plan, that advances the understanding of drug abuse among PLWH, and facilitates studies of drug abuse and testing, engagement, treatment, and retention in care. We are ideally suited to lead the STTR DCC, bringing a background of clinical HIV research including substance use, expertise in biostatistics, epidemiology and project coordination, and vast experience with data harmonization for cohorts and studies of PLWH. We will test and advance emerging methods for integrating, analyzing, and interpreting harmonized large data sets; serve as a centralized resource to facilitate and support research site activities; and conduct cross-site analyses using the STTR integrated dataset. In consultation with NIDA and the study PIs, we will lead a strong research agenda focused on the clinical questions that requires the power of a consortium of studies to successfully elicit answers to generate new knowledge on the intersection of HIV, drug abuse and the HIV treatment cascade to inform future research and clinical care in the US and globally.